When an internal combustion engine is operated in the HCCI (Homogeneous Charge Compression Ignition) operating mode, which is sometimes also referred to as CAI (Controlled Auto Ignition), ATAC (Active Thermo Atmosphere Combustion) or TS (Toyota Soken), the air-fuel mixture is ignited not by externally applied (spark) ignition, but by controlled self-ignition.
The HCCI combustion process can be triggered for example by a high proportion of hot residual exhaust gases and/or by a high compression and/or a high inlet air temperature. A prerequisite for the self-ignition is a sufficiently high energy level in the cylinder. Internal combustion engines which can be operated in the HCCI operating mode, which engines may be spark ignition engines or diesel engines, are known; see, for example, U.S. Pat. Nos. 6,260,520, 6,390,054, DE 199 27 479 and WO 98/10179.
Compared with a conventional combustion process based on externally applied (spark) ignition, HCCI combustion has the advantage of reduced fuel consumption and lower exhaust emissions. The internal combustion engine is therefore operated largely in the HCCI operating mode. However, in certain operating ranges such as, for example, at high engine speeds or under heavy load this is not possible, so that it is then necessary to switch over to an operating mode using externally applied (spark) ignition, in particular to a homogeneously stoichiometric mode of operation. The switchover between said operating modes is performed taking into account variables such as fuel consumption, exhaust emissions and drivability as a function of corresponding operating parameters such as, for example, load and engine speed.
During the operation of the internal combustion engine, however, exceptional situations can arise in which this customary “switchover strategy” leads to a disadvantageous mode of operation.